Protective circuits



Nwz 7,1939. Wl J, PQCH 2,178,764

PROTECTIVE CIRCUITS Filed May 26, 1956 loo, ooo* loo,

. Zmnentor Walaczfzarcg/ (Ittorneg Patented Nov. 7, 1939 UME STAT PROTECTIVE CIRCUITS of Delaware Application May 26,

5 Claims.

My invention relates to protective circuits for thermionic devices and particularly to protective circuits for cathode ray tubes used in television receivers.

In a preferred type of television receiver employing a cathode ray tube, the control grid of this tube is connected by a direct current connection to the plate of the last amplier tube whereby the last amplier stage will amplify direct current so that automatic control of the picture background may be obtained. In such a circuit the negative ias for the control grid of the cathode ray tube is obtained from the voltage drop in the plate circuit of the above mentioned amplier tube. Since the cathode of the amplier tube ordinarily is of the indirectly heated or equi-potential type there is an appreciable interval after the power has been supplied to the receiver before plate lcurrent iiows through the amplifier tube and its plate circuit. During this warm up period, the cathode ray tube control grid is not maintained at a negative potential unless special precautions are taken, and the cathode ray tube may be injured as a result.

Accordingly, it is an object of my invention to provide an improved circuit whereby injury to a cathode ray tube or the like during the warm up period is prevented.

More specically an object of my invention is to provide in an amplier of the direct current type a protective circuit which will maintain the control grid of a therrnionic device negative irnznediately after power has been applied to the circuit.

In practicing my invention I employ a. circuit of the general type described and claimed in Patent No. 2,092,373 issued Sept. '7, 1937, in the name of Ralph S. Holmes and assigned to the Radio Corporation of America. As in the circuit described in the Holmes application, I include a thermionic device in the grid-cathode circuit of the cathode ray tube to interrupt this circuit, so far as the iiow of direct current is concerned, during the warm up period. In accordance with my invention, howeverJ7 I so designed the circuit that not only does the control grid of the cathode ray tube never go positive, but it is biased negatively at all times that power is applied to the circuit.

My invention will be more fully understood by referring to the accompanying drawing in which, Figure l is a circuit diagram of one embodiment of my invention, and

Fig. 2 is a circuit diagram of a second embodiment oi my invention.

1936, Serial No. 81,809

Referring to Fig. 1, my invention is shown applied to a television receiver including a cathode ray tube l and an amplifier tube 2. The cathode ray tube I is of conventional design comprising an indirectly heated cathode 3, a control grid d, 5 a first anode a second anode I and a fluorescent screen 8. The amplier tube 2 is of a well known type including an indirectly heated cathode Il, a control grid I2, a plate I3, and a diode plate I4 which has for its cathode a portion of the main cathode II.

The input circuit of the amplier tube 2 includes a grid condenser I6 and a grid resistor I'I which are so adjusted that incoming synchronizing impulses produce a biasing potential across,15 the grid resistor. The magnitude of this negative bias varies in accordance with the varying height of incoming synchronizing impulses whereby the flow of direct current through the plate circuit of the amplifier tube Z is a measure of the picture background. The varying voltage dropcaused by this change in direct current ow appears across the plate resistor I8 and is applied to the control grid I of the cathode ray tube by a direct current connection I3. An automatic background control circuit operating in this way is described and claimed in application Serial No. 718,192, led March 30, 1934, in the name of Waldemar J. Pooh and assigned to the Radio Corporation of America. 30

Voltage for the plate of the amplifier tube 2 is supplied from any suitable direct current source such as the voltage divider 2i which is connected at one end to ground and tothe negative terminal of a rectier-lter unit (not shown) and connected at the other end to the positive terminal oi this rectifier-filter unit.

A shunt circuit is connected across` the voltage divider 2i, this shunt circuit including a potentiometer resistor 22 connected at one end to the positive end of the voltage divider 2l and connected at the other end through a resistor 25 and a conductor 23 to the diode plate I4. The shunt circuit is completed through the diode III- Id to ground and through ground to the negative end of the voltage divider 2l. The cathode 3 of the cathode ray tube is connected through a variable tap 24 to a point on the shunt resistor 22. The cathode 3 is also provided with an alternating current connection to ground through a by-pass condenser 26.

In accordance with my invention, the plate I3 of the amplifier tube is connected through the plate resistor I8 and the usual peaking coil 21 to a point 28 on the voltage divider 2l which is 55 been indicated on the drawing in ohms.

negative with respect to the point 29 on the voltage divider to which the potentiometer resistor 22 is connected. It will be seen that, with this connection, immediately after power has been applied to the receiver, and before the cathode l! of the amplifier tube has become heated, the only voltage applied to the control grid 4 of the cathode ray tube is the voltage drop across the voltage divider between the points 28 and 2S, this voltage drop biasing the control grid l negatively. During the warm up period, therefore, the intensity of the cathode ray is held below a value which would cause injury to the fluorescent screen or to other parts of the cathode ray tube,

As soon as the amplier tube 2 warms up, plate current flows through the plate resistor H3 and peaking coil 2l to apply a further negative bias to Ythe control grid Il of the cathode ray tube and current ows through the shunt resistor 22 and diode tending to impress a positive voltage upon the contro-l grid il; thus by adjusting the variable tap f/t'l on the resistor 22 the negative bias on the control grid ll may be adjusted to the desired value.

The resistor is included between the potentiometer and the diode plate lll in order that the slider cannot be moved to a position where a positive voltage is applied to the grid 4. In the circuit illustrated, the grid il is about volts negative even though the slider 2Q is in its most positive position.

It may be noted that, when the protective diode is in the same envelope with the amplifier electrodes, there is no danger of injuring the cathode ray tube by removing the last amplifier tube while the power is on since 'this breaks both the amplifier plate cir uit and the shunt potentiorneter circuit.

In Figure 2 there is shown a circuit which, like the circuit shown in Fig. l, maintains the control grid il ol" the cathode ray tube negative at any time that power is applied to the receiver. In the two like parts are indicated by the same reference numerals. In this embodinient of the invention the plate lfd of the ampliier tube 2 is prerably connected to the most positive point on the voltage divider 2l whereby grid .1 of the cathode ray tube would negative bias during the warm up Llod if the saine circuit connections as those own in l were employed. l overcome this di...;culty by connecting a resistor 3i between the direct current connection lil and ground whereby, during the wann up period, current flows from the positive terminal of the voltage divider *i rough the plate resistor lil and the addiio resistor Si to ground, and through ground o ne gative end of the voltage divider 2l. Since at time there is no current ow through the diode section of the amplifier tube Si, the cathode 3 of the cathode ray tube is at the potential as the lower end of the peaking coil El'. It will be apparent, therefore, that the flow of current through the plate resistor i2 and pecking coil 2l during the warm up period applies a negative potential to the control grid l whereby damage to the cathode ray tube is prevented` The resistance of certain circuit elements has These values are given merely by way of example and may be varied within wide limits.

From the foregoing description it will be apparent that various modicaticns may be made in rny invention without departing from the spirit and scope thereof and I desire that only such limitations shall be imposed thereon as are necessitated by the prior rt and are set forth in the appended claims.

I claim as my invention:

l, Electrical apparatus comprising an amplifier tube having a cathode and a plate, an electric discharge tube having a cathode and a control grid, a direct conductive connection between said plate and said control grid, a source of direct current havinfr positive and negative terminals, a circuit in shunt to said source, said circuit including a resistor having one end connected to said positive terminal and having in series therewith a device through which current flows readily only when heat is applied thereto, said device having at least as much thermal time lag as the cathode of said aniplier, a VdirectY current connection between the cathode of said electric discharge tube and point on said roa connection between the cathode of said amplifier tube and said negative terminal, and a plate resistor connected between the plate of said amplifier tube and a point on said source which is less positive than said positive terminal.

2. in combination, a cathode ray tube having a control grid and a cathode which is electron `1ive when heated, an electric discharge tube ha .fing an anode and a cathode which is electron emissive when heated, a direct conductive conn ection between said anode and said control grid, voltage divider .having positive and negative Aals, a connection between said negative ternjnal and the cathode or" said electric discharge tube, a resistor, means for applying potenti-al to said anode through said resistor from nt on s-id voltage divider which is less positive than said positive terminal whereby a plate current ow through said electric discharge tube produces a potential drop across said resistor, a resistor connected between the cathode of said cathode ray tube and said positive terminal, a device through which current flows readily only when heat is applied thereto, said device having at least much thermal time lag as the cathode of said discharge device, and a connection between cathodes which includes said last device.

3. ln combination, cathode ray tube having a control grid and cathode which is electroneniissive when heated, an amplifier tube having an anode and a cathode which is electron-emissivc when heated, a direct conductive connection beJwcen said plate and said control grid, a voltage divider having a positive terminal and a negative terminal, a c cuit in shunt to said vo ge divider, circuit including a resistor having one end connected to said positive terminal and having in series therewith a device through which current flows readily only when heat is applied thereto, said device having at least much thermal time lag as the cathode of said amplifier, a direct current connection between the cathode of said cathode ray tube and a point on said resistor, a connection between said negative terminal and the cathode of said amplifier tube, and a plate resistor connected between the plate of said ampliiier tube and a point on said voltage divider which is less positive than said positive terminal.

li. The invention according to claim 3 characterized "n that said device is a diode having an indirectly heated cathode.

5. ln combination, cathode ray tube having a control grid and a catho-de which is electronemissive when heated, an. amplifier tube having a plate and a cathode which is electron-emissive when heated and also having an auxiliary diode plate, a direct conductive connection between said rst plate and said control grid, a voltage divider having positive and negative terminals, a circuit in shunt to said voltage divider, said circuit including a resistor1 having one end connected to said positive terminal and having the other end connected to said diode plate, a direct current connection between the cathode of said cathode ray tube and a point on said resistor, a connection between the cathode of said amplifier tube and said negative terminal, and a plate resistor connected between the plate of said amplier tube and a point on said voltage divider which is less positive than said positive terminal.

WALDEMAR J. POCH. 

